Economizer based control schemes used in commercial air conditioning equipment take advantage of what is referred to as "free" cooling. In other words, when external air may be used for cooling, it is desirable to control a damper to adjust the amount of exterior air being utilized. Some of these economizer systems use dry bulb temperature to decide if the outdoor air is suitable for use. Other systems use enthalpy as a basis for this decision. A system using the enthalpy of the air is more efficient than one relying only on temperature, because enthalpy is a measure of the energy in the air. Accordingly, it is desirable to provide a device to calculate enthalpy from temperature and humidity.
Some prior art systems which use both temperature and relative humidity to control a damper utilize a mechanical humidity sensor. The mechanical humidity sensor is connected to a temperature sensing element and a mechanical link which actuates a mechanical switch.
Another prior art enthalpy sensor utilizes an electronic humidity sensor and an electronic temperature sensor. The output of the sensors are provided to a digital circuit and a look-up table is accessed, thereby obtaining an enthalpy value for the sensed values. However, the accuracy of a look-up table is limited by its size and provides a "stepped" output rather than a continuous output, and the cost of a look-up table increases with size. Accordingly, an enthalpy calculator that has a look-up table to determine enthalpy is not useful over a wide range of temperature and humidities, because either the cost of the look-up table will be prohibitive or the enthalpy calculations will have a significant error.
Another prior art system described in U.S. Pat. No. 4,672,560, issued Jun. 9, 1987, to Katayama et al., discloses an enthalpy calculator which provides a circuit to model an enthalpy calculation. However, the enthalpy calculation modeled therein is a linear approximation of a more precise equation. Accordingly, the approximation and the circuit implementing the approximation are limited by their range of use and accuracy. For example, as described in U.S. Pat. No. 4,672,560, the range of temperatures the equation is accurate for is from 20.degree.-30.degree. C. Such a range is not broad enough for use in a system where the enthalpy will be used to determine whether or not to control a damper and utilize outside air.
In view of the prior art deficiencies noted above, it is desirable to provide an enthalpy calculator that provides a continuous output, rather than a stepped output such as that from a look-up table. Preferably the enthalpy calculator will model an equation to provide an enthalpy value that is accurate over a wide range of temperature and humidity. Thus, the enthalpy calculator should model a substantially exact equation rather than merely a linear approximation.